1. Field of the Invention
The present invention relates to an image processing apparatus for outputting halftone images having improved resolution and gradation.
2. Brief Description of the Prior Art
A scanning system having a rotary polyhedral mirror or an oscillating mirror has been widely adopted in various types of like facsimiles, display devices, recording devices and the since the scanning angle can be made large, the color dispersion is small, and so on. In particular, a rotary polyhedral mirror has been widely adopted for high-speed scanners.
As a conventional method for recording or displaying halftone images with a scanning system of the type described above, there is known a method for reproducing halftone images by dividing one picture element on a recording or displaying surface into a matrix of n.times.n (where n is a positive integer) micropicture elements and by appropriately arranging white micropicture elements and black micropicture elements in the matrix. FIGS. 1(A) to 1(E) show examples of matrices according to such a recording system. According to this recording system, a picture element 11 consists of 2.times.2 micropicture elements 13. In order to obtain halftone images with such a picture element configuration, images of five sequentially increasing gradations may be obtained by sequentially increasing the number of micropicture elements to be displayed in black as shown in FIGS. 1(A) to 1(E). In general, with a micropicture element matrix of n.times.n micropicture elements, it is possible to obtain images of (n.times.n+1) gradations. According to this system for reproducing halftone images, since micropicture elements need only be recorded as binary values of white and black, the overall system may be of simple construction. Furthermore, since the gamma of photographic emulsion, the ratio of the output density to the input light amount can be nonlinear, the kind of photosensitive body that can be used is not limited. On the other hand, according to this system, since one picture element consists of a plurality of micropicture elements, the size of the individual picture element is greater, resulting in lower resolution of an image. In order to solve this problem, it is necessary to decrease the size of the picture element. The size of the micropicture elements then must be made extremely small for this purpose. When this system is adopted for a laser beam printer, for instance, the diameter of the beam spot on a photosensitive drum must therefore be very small. Such a requirement may be met with an optical system. However, satisfactory response often may not be obtained with micropicture elements of such a small size because of the characteristics of the photosensitive body.
FIG. 2 is a graph showing the characteristics of a photosensitive body wherein the space frequency is plotted along the abscissa and the MTF (modulation transfer function) value is plotted along the ordinate. These characteristics correspond to a photosensitive body for electrophotography. Curve 21 represents the electrostatic latent image characteristic, while curve 23 represents the development/transfer characteristic. As may be seen from these characteristics, the higher the space frequency, the lower the MTF value. For this reason, image outputs of satisfactory gradation cannot be obtained with the halftone image reproducing system as described above which requires correct response from individual micropicture elements. When the size of the picture element is made smaller for actual recording according to this system, the individual micropicture elements are not completely recorded in black. Then, an increase or decrease by one of micropicture elements which are recorded in black is not reflected in the output. For this reason, with the system as described above, when the size of the picture elements is made smaller for the purpose of improving the resolution, gradation is impaired. On the other hand, when the size of the picture element is increased for the purpose of improving gradation, the resolution is degraded.